Endoluminal stenting has provided a major advancement in clinical treatment modalities offering a significant reduction in perioperative treatment times, iatrogenic injury, postoperative morbidity and healing times. Even with the unprecedented clinical advantages of these devices, there still remains a number of limitations and disadvantages of the technologies currently available. The two primary technologies available for endoluminal stenting are the use of bare metal stents and stent devices provided with a covering or lining of a tubular graft material, i.e., stent-grafts. Either of these technologies may be made to be deployed via inflation of a catheter balloon (e.g., stainless steel stents) or to be self-expanding (e.g., nitinol stents). All of these technologies exhibit a common disadvantage in that none of the commercially available devices are designed to be removable after implantation.
There are numerous applications for which a removable stent-graft would be highly desirable. Even though great strides have been undertaken to enhance biocompatibility of these devices, it is still a synthetic, non-living tissue device that constitutes a foreign body. As a result, living tissue has a number of limitations and/or reactions in coping with such a foreign body.
The most common of these is infection. Typically, when a synthetic device becomes infected, or colonized by bacteria, there is little success in resolving such an infected device or infected area short of device removal from the patient. In some instances, if an infected synthetic device cannot be removed enabling the antibiotic treatment of the affected living tissue, patient mortality can result due to septic shock.
Another issue associated with implantation of endoluminal stents and stent-grafts is foreign body reaction. Endoluminal stents and stent-grafts are often employed to limit, or control, the body's normal healing response (restenosis) to vascular, luminal, or ductal injury due to balloon dilatation. Even though these devices aid in limiting the amount of restenosis as a result of vessel or ductal injury, after a period of time the vessel or duct may generate a hyperplastic tissue (restenotic) or calcific stone formation response due to the presence of the foreign body. Consequently, removal of the device after the appropriate therapeutic period may be desirable.
Still another application for a removable stent-graft would involve providing a removable support structure for delivery of certain other implantable materials or structures (e.g., tubular structures) which otherwise do not exhibit the necessary mechanical characteristics for device delivery without the aid of a temporary, supporting stent component.
Further, mechanisms for localized drug delivery continue to be a highly sought after treatment option which offer many advantages over systemic drug delivery. Two of the key challenges in local drug delivery are the delivery mechanism and the drug elution profile or therapeutic window of the drug delivery. These are not unusually interrelated. By employing one or more applications of a removable drug eluting stent-graft, therapeutic windows can be greatly increased providing unlimited drug application profiles.
Thus, the array of clinical treatments modalities for such a removable endoluminal stent-graft includes: malignant and benign strictures of the biliary tract due to tumor compression, anastomotic and bile stone nidus; anastomotic and benign strictures of the colon, small intestine and ureter/urethra; esophagus collapse syndrome and gastric reflux erosion; strictures of the tracheal/bronchial tree; treatment of vascular disease or injury; and localized drug delivery for various chemotherapy application.
Various designs for removable stents are known in the art. For example, Myler et al. in U.S. Pat. No. 5,474,563 describe a retrievable stent and retrieval tool. The described stent is removed intact, at its fully deployed dimensions, and may consequently pose a risk of trauma during removal.
U.S. Pat. No. 5,782,903 to Wiktor et al. describes a removable stent system which comprises a continuous serpentine wire formed into helical coil. The coil after implantation can be uncoiled by use of a retrieval line. Beyar et al. in U.S. Pat. No. 6,090,115 also describe a temporary stent system comprising a stent constructed of a helical coil of biocompatible material. Both of these references teach that the stent is not covered (i.e., is not a stent-graft) and therefore provides opportunity for tissue in-growth into the spaces between the coil structure over time. This in-growth may result in trauma to the implant site during retrieval.
U.S. Pat. No. 5,799,384 to Schwartz et al. teaches a stent similar to the above-described Wiktor et al. stent. It differs from the Wiktor et al. stent in that a tape of polymeric film is provided to the stent wire, the length of the tape running parallel to the length of the wire with the width of the tape being centered over the stent wire and therefore extending perpendicularly from the stent wire a short distance from both sides of the wire. When the wire is wound into a helical form to create a stent structure, the polymeric tape provides a sort of graft covering. However, this graft covering is discontinuous and therefore cannot offer the advantages of a continuous graft covering extending for all or a major portion of the length of the implantable stent structure.
Huxel et al. in U.S. Pat. No. 6,494,908 describe a removable stent in the form of a helical winding wherein adjacent windings are in direct contact; removal is accomplished by grasping an end of the helix and unwinding the helical form. The helical form of the Huxel et al. device is made from a soft, flexible fiber that is provided with an outer coating of a bioabsorbable material to render it rigid for insertion into a body conduit. The device becomes thinner and flexible over time in order to allow the stent to be removable after a pre-determined time has passed. U.S. Pat. No. 5,514,176 to Bosley et al. teaches a somewhat similar device in the form of a removable stent-graft made from a series of helical windings with the adjacent windings in contact with each other. An exterior coating of silicone is provided to seal between the adjacent windings. Removal is accomplished by unwinding the device whereby the coating is removed simultaneously with the helical winding.
Camrud et al., in U.S. Pat. No. 6,258,117, teach a multi-section stent which incorporates a connecting structure that can separate. This ability to separate adjacent segments of the connecting structure however is promoted as a means to add flexibility to the implanted device rather than as a way to atraumatically remove portions of it. Removability of the segments is not taught or suggested. Iwasaka et al. in US Patent Application Publication No. 2003/0114922 describe a stent-graft having a series of discrete, ring-like stent structures along its length. The device is removed from a body conduit by grasping its distal end with a retrieval device and everting it from the distal end by pulling it through itself in a proximal direction. The device is removed in its entirety rather than being removed segmentally.
WO00/42949 teaches the construction of an impermeable stent-graft that is primarily intended for biliary applications. This stent-graft is not described as being removable.